I want to add curl and jansson to my c9 c++ project. I can't find any c9 documentation for this task so is this even supported?
Think of Cloud9 workspaces like Ubuntu machines that you have sudo access to. They come with a lot of libraries pre-installed, but it shouldn't stop you from installing and using most other libraries.
Since each workspace is uses Ubuntu, installation instructions for Ubuntu should work as-is on Cloud9 workspaces.